| Mg-Zn alloys have the potiential for developing newly heat-resistant magnesium with low-cost and high-performance. Mg2Si phases with high melting point (1085℃) precipitation will increase the heat-resistant porperties When Si element adding into Mg-Zn-Al alloys, respectively, the Chinese script-type Mg2Si phases are prone to be the points for strain concentration and it is not conducive to improvement of the mechanicil.In this thesis, the influences of different alkaline earth metals Ca and Sr additions on microstructure and mechanical properties of ZAS8309 based alloy are systemically studied by using measurements of Optical Mierostructure(OM), X·ray Diffaction(XRD), Scan Electric Microscopy(SEM) with Energy Dispersive Spectrometer(EDS), Differential Thermal Analyzer(DTA), RST5000 electrochemical workstation, Corrosion on weight loss, Tensile test at room temperature and 175℃.The composition design of experiment alloys are Ca (0.15%,0.3%,0.45%,0.6%, 1.0%), Sr(0.05%,0.1%,0.15%,0.2%) and Ca(0.15%,0.3%,0.45%)-Sr(0.05%,0.1%, 0.15%). The experimental alloys were prepared by gravity casting.The results show that the as-cast microstructure of ZAS8309 alloy consists of a-Mg matrix,τ-(Mg32(Al, Zn)49) phase and Mg2Si phase.Alloy elements did not the consists of the phases significantly. CaSi2 phases were found when 1.0%Ca and 0.45Ca-0.15Sr addition. Additional, a few of loose ternary phases showed in the SEM graph. The matrix grain were refined from 90～100μm to 75μm and keeped stable grainsize range when Ca, Sr and Ca/Sr additions, this satituation was benefit to keep stable quantities of grain-boundaries and then reduct the slipping possibility. The tenary phasesτ-(Mg32(Al, Zn)49) keeped semi-continuous bone-like morphology and distributed along grain-boundaries.The chinese script-type Mg2eSi phase have been modified to round granular morphology or polygonal particles obviously when 0.3%Ca addition into the base alloy,and that, the size Of Mg2Si phases been modified were stable at about 10μm. This reason is the particles CaSi2 provided enough heterogeneous cores to Mg2Si phases. When 0.05～0.15% Sr added into the base alloy, chinese script-type Mg2Si phase changed to polygonal particles essecially with 0.1% Sr addition.This sitiuation due to the surface active element Sr. Firstly, it obstructed the diffusion of atom Mg and Si; Secondly, it caused the constitutional supercooling in front of Mg2Si phases solidification and increase the nucleation rate of Mg2Si phases.When Ca/Sr added into the base alloy, Function together modified the chinese script-type Mg2Si phase to polygonal particles by Ca-element and Sr-element, for example, the lower Ca-content wit Sr:0.15Ca-Sr. It is reported that lower Ca-content be good at the casting performance of magnesium alloy.Tensil test results showed that:the ultimate tensile strength first increased and then decrease both at room temperature and 175℃with the incease of Ca and Sr-content.At room temperature, The maximum ultimate tensile strength are 176MPa(in the Ca-contain alloys with 0.3%Ca addition),197MPa(in the Sr-contain alloys with 0.1%Sr addition) and 145MPa(in the Ca/Sr-contain with 0.15Ca-0.O5Sr addition); At 175℃, The maximum ultimate tensile strength are 141MPa,178MPa and 132MPa in turn. These results mainly due to the chinese script-type Mg2Si phase have been modified, additional, the Sr-element may make the alloy more pure than Ca-element by occupying the cavities and stomaes on grainboundary, thus, the alloy with Sr had the most maximum ultimate tensile strength. The fracture characteristic of experiment alloys were intergranular brittle fracture mainly by SEM graphs.The microhardness of 03Ca, 01Sr,015Ca01Sr alloys at 150℃and 200℃(12h, 24h,48h,96h) results showed that a certain extent decrease at elevated temperature, howere, the microhardness of alloys kept at a high level, for example, the microhardness values of experiment alloys are more than 70HV0.1 at 200℃/96h. The above sitiuation showed the experiment alloys keep stable heat-resistant performance.The DTA results shoowed that liquidus decrease and solidification range shortened with Ca, Sr and Ca/Sr addition, the 0.1%Sr addition showed the signfigcant result.The liquidity of alloy may be increased by the above sitiuation.Corrosion experiment result indicated both corrosion current density and corrosion rate of weight loss decrease with with Ca, Sr and Ca/Sr addition. Alloy with 0.1%Sr addition showed the best corrosion resistance and mainly due to the reduction of contact area between Mg2Si phases and matrix phases by the Chinese script-type Mg2Si phase modifition. |
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